1. Field of the Invention
The present invention relates to an IEEE 1394 apparatus having an interface complying with the IEEE 1394-1995 standards (hereinafter referred to as IEEE 1394 standards), and more particularly to a power class control for controlling power class information.
2. Description of the Related Art
An interface complying with the IEEE 1394 standards attracts attention as a high-speed interface of the next generation. IEEE 1394 standards prescribes that 63 nodes at maximum can be connected to the same bus. Each node is not required to have its own power supply, and any node that does not have its power supply is adapted to receive power from the serial bus and operates with the power. For this reason, the power of the entire serial bus must be managed by some method so that the total power consumed by those nodes that receive supply of power from the serial bus may not exceed the power that can be supplied through the serial bus.
One of the nodes connected to the serial bus serves as a bus manager to manage the power of the serial bus. Therefore, the bus manager must acquire information on power in W (watt) each of the nodes connected to the serial bus can supply power to the serial bus when the node is operating with a power supply (power supply of its own) provided in the node itself and information on power in W each node consumes when the node is operating with power supplied through the serial bus.
To this end, each of the nodes connected to the serial bus performs self-identification to determine whether it is operating with a power supply of its own or with power supplied through the serial bus, when bus reset to initialize the serial bus has occurred. Each node then includes the result of determination as power class information in a Self-ID packet to output the Self-ID packet to the serial bus. The bus manager of the serial bus manages the power of the serial bus based on the power information in Self-ID packets output from the nodes.
Each node having an interface complying with IEEE 1394 standards has a power class controlling function to generate and control a code representative of such power class information.
A node denotes an apparatus that outputs a Self-ID packet to the serial bus and is recognized by the serial bus. However, one IEEE 1394 apparatus does not necessarily serve as one node, but may include a plurality of nodes. For the sake of simplicity, the following description is given of a case wherein an IEEE 1394 apparatus includes only one node. Thus, an IEEE 1394 apparatus means a node.
In order to describe a conventional IEEE 1394 apparatus having a power class controlling function, a 2-port repeater for connecting two ports is shown by way of example. FIG. 1 is a system block diagram showing a connection of a serial bus in which nodes 40 and 41 are interconnected with 2-port repeater 102. 2-port repeater 102 has two serial bus connectors 9, 10, which are connected to nodes 40, 41, respectively.
A configuration of conventional 2-port repeater 102 is shown in FIG. 2. 2-port repeater 102 includes power supply circuit 2 for supplying power to 2-port repeater 102 itself and the serial bus, diode 3, DC—DC converter 4, physical layer circuit (hereinafter referred to as PHY) 5, and power-on reset circuit 11.
Diode 3 prevents reverse current of power supplied from the serial bus. DC—DC converter 4 converts a voltage supplied from the serial bus or power supply circuit 2 into a voltage necessary for itself. Power-on reset circuit 11 generates a reset signal when it detects that a voltage has been supplied to PHY 5.
A power supply voltage supplied from power supply terminal 1 is converted into a DC voltage necessary for the serial bus by power supply circuit 2 and is supplied to the serial bus via diode 3 through serial bus connectors 9, 10. The voltage supplied to the serial bus is converted into a voltage necessary for PHY 5 by DC—DC converter 4 connected to the cathode side of diode 3 and is supplied to PHY 5. On the other hand, when no input voltage is supplied to power supply voltage input terminal 1, a DC voltage output from another node is supplied to DC—DC converter 4 through serial bus connector 9 or 10 and converted into a voltage of a predetermined value by DC—DC converter 4, and the voltage of the predetermined value is supplied to PHY 5. In this instance, the provision of diode 3 prevents a DC voltage from the serial bus from flowing reversely to power supply circuit 2.
Serial bus connector 9 or 10 supplies a DC voltage and a signal on the serial bus to another port. The DC voltage is supplied from connector pin 91 (101) directly to connector pin 101 (91). The signal is supplied from connector pin 92 (102) to connector pin 102 (92) past PHY 5.
When a voltage from DC—DC converter 4 is applied to PHY 5, a reset signal is output from power-on reset circuit 11 to reset PHY 5. The reset of PHY 5 once disconnects 2-port repeater 102 from the serial bus, and after the resetting is completed, 2-port repeater 102 is connected to the serial bus again. Reset of PHY 5 and the change of connection of the serial bus causes the nodes (nodes 40, 41 in FIG. 1) confronting with 2-port repeater 102 to detect the change of the connection, occurring bus resetting to initialize the serial bus.
In a system employing a serial bus, if a configuration of the serial bus is changed, a node that has detected the change generates bus resetting. The change of configuration of the serial bus includes a new connection of a node to the serial bus and a disconnection of a node from the serial bus.
In a system employing a serial bus, after bus resetting is performed, tree identification to identify a connection relationship in the system is performed, and then self-identification is performed in each node, the result of which is output as a Self-ID packet to the serial bus.
When the self-identification is performed, in 2-port repeater 102 in FIG. 2, power class information is previously set with manual operation and PHY 5 reads the power class information, includes it into a Self-ID packet and outputs the Self-ID packet to the serial bus. The bus manager in the serial bus checks the power class information in Self-ID packets to manage the power of the serial bus.
In 2-port repeater 102 which is a conventional IEEE 1394 apparatus, PHY 5 cannot determine whether the apparatus operates with power from power supply circuit 2 or with power supplied from the serial bus. For this reason, 2-port repeater 102 is unable to set a power class corresponding to its operation state when performing self-identification. This requires the aforementioned conventional IEEE 1394 apparatus to set a power class with manual operation.
FIG. 3 shows 2-port repeater 103 as an example of an IEEE 1394 apparatus that eliminates the necessity for such manual operation and can automatically set a power class. 2-port repeater 103 of FIG. 3 includes voltage detection unit 6 and code generation unit 7 in addition to the components of 2-port repeater 102 of FIG. 2.
Voltage detection unit 6 detects a state of an output voltage of power supply circuit 2 and determines whether or not a DC voltage is applied to an output terminal of power supply circuit 2. More specifically, voltage detection unit 6 is comprised of a comparator or the like and compares the DC voltage at the output terminal of power supply circuit 2 with reference voltage Vref to determine whether or not power supply circuit 2 has an output voltage.
Code generation unit 7 generates a code indicative of power class information representing that 2-port repeater 103 operates with a power supply of its own when voltage detection unit 6 has determined that an input voltage is applied to power supply input terminal 1, and generates another code indicative of power class information representing that 2-port repeater 103 operates with power from the serial bus when voltage detection unit 6 has determined that an input voltage is not applied to power supply input terminal 1.
In the conventional IEEE 1394 apparatus, voltage detection unit 6 detects presence or absence of an output voltage from power supply circuit 2, and code generation unit 7 generates a code representative of power class information based on the result of detection. This obviates manual operation for setting power class information.
In the conventional IEEE 1394 apparatus, when the power to be used is supplied either from power supply circuit 2 or from the serial bus, the state of the power supply coincides with the power class. However, the conventional IEEE 1394 apparatus suffer from a problem that when the used power supply is changed over after it starts operation with one of the power supplies, for example, when the power supply of its own is turned on while the apparatus is operating with power from the serial bus and consequently the apparatus now operates with the power supply of its own, the actual operation state does not coincide with the power class recognized by the bus manager.